1. Field of the Invention
The invention relates to a display device, in particular a cathode ray tube, comprising a display screen having an antistatic, light-absorbing coating which contains latex particles of electroconductive polypyrrole.
The invention also relates to a method of manufacturing an antistatic, light-absorbing coating on a display screen.
2. Description of the Related Art
Antistatic coatings are used on display screens of display devices, in particular cathode ray tubes (CRTs). Such coatings are sufficiently electroconductive to ensure that a high electrostatic voltage present on the outside surface of the display screen is removed within a few seconds. Thus, the user does not experience an unpleasant shock if he touches the screen. Besides, the attraction of atmospheric dust is reduced.
Light-absorbing layers which reduce the light transmission are used on display screens of display devices, for example cathode ray tubes and liquid crystal display devices (LCD and LC-TV), to improve the contrast of the image reproduced. By virtue thereof, the necessity of changing the glass composition of the display screen is avoided and the possibilities of bringing the light transmission to a desired value in a simple manner are increased. Such filtering layers reduce the transmission of incident ambient light as well as light originating from the internal light source, for example the CRT phosphors. The incident ambient light passes through the light-absorbing coating and the glass display screen, subsequently, reflects off the rough phosphor layer on the inside of the display screen and again passes through the display screen and the light-absorbing coating. If the transmission of the light-absorbing coating amounts to T, then the intensity of the reflected ambient light is reduced by a factor of T.sup.2. Light originating from the CRT phosphors passes only once through the light-absorbing coating, so that the intensity of this light is reduced only by a factor of T. The combination of these effects causes an increase of the contrast by a factor of T.
In published European Patent Application EP-A-533256, assigned to the present assignee, a description is given of the manufacture of an antistatic, light-absorbing coating which comprises a layer of a matrix of silicon dioxide and latex particles consisting of an electroconductive polypyrrole compound. In the known method, an aqueous dispersion of polypyrrole latex particles is added to an aqueous solution of a hydrolysed alkoxy silane compound, whereafter the dispersion is provided on the display screen and the hydrolysed alkoxy silane compound, for example tetraethyl orthosilicate, is converted to silicon dioxide by means of a treatment at temperatures between 150.degree. C. and 170.degree. C. for at least 30 minutes. The polypyrrole dispersion is prepared in the presence of an excess of water-soluble polymer as the steric stabilizer, for example polyvinyl alcohol. Since conductive polypyrrole is black, the coating obtained also has light-absorbing properties and a neutral grey exterior.
Since polypyrrole is an electroconductive material, a coating on the basis of a polypyrrole dispersion would result in an electroconductive layer. The presence of an excess of a steric stabilizer, however, prevents the layer from being electroconductive. In the known method, the coating is rendered electroconductive (antistatic) by removing the excess stabilizer by a centrifuging treatment for 1 hour at 11000 r.p.m., whereafter the polypyrrole particles are again dispersed in water. This method is time consuming due to the additional process steps.
If desired, the known coating can be supplemented with layers having an antireflective or antiglare effect or with layers which improve the scratch resistance. These additional layers are usually provided by spinning or spraying of an alcoholic alkoxy silane compound. Therefore, it is often necessary to subject the first coating formed from the aqueous mixture of the polypyrrole dispersion and the alkoxy silane compound to a thermal drying treatment to preclude that this layer is washed away when the additional layer is provided.